Prosthesis for simulating natural kinematics

ABSTRACT

A bearing component  2  for a joint replacement prosthesis comprises a first bearing element  4 ; a second bearing element  6 , and a linking element  8 , operatively connecting the first and second bearing elements  4, 6  and permitting relative motion there between. The flexible linking element  8  prevents dislocation of mobile bearings in a total knee replacement prosthesis. The invention also relates to a bridging element which retains the linking element  8  with some play, which acts as a ligament support  2051 , and which causes a deflection of the line of action of a ligament  1018.    
     A joint replacement prosthesis is also disclosed comprising a biasing element  1140  or a tensioning element  1220  operatively coupled to the artificial ligament  1018 . The biasing element  1140  or tensioning element  1220  may be housed in the stem of a tibia tray  1006.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of: (1.) U.S. patent application Ser.No. 13/923,768 filed on Jun. 21, 2013, and (2.) U.S. patent applicationSer. No. 13/923,779 filed on Jun. 21, 2013, which is a continuation ofU.S. patent application Ser. No. 12/740,998 filed Nov. 23, 2010, nowU.S. Pat. No. 8,470,048 issued on Jun. 25, 2013, which is a 371 U.S.National Phase Application based on International Application NumberPCT/GB2008/003677 filed Oct. 31, 2008, which claims the benefit of: (1.)Great Britain Patent Application No. 0721610.4 filed Nov. 2, 2007, and(2.) Great Britain Patent Application No. 0805917.2 filed Apr. 1, 2008.The entire disclosures of each of the above applications areincorporated herein by reference.

FIELD

The present disclosure relates to a prosthesis for simulating naturalkinematics and particularly, but not exclusively, relates to a bearingcomponent and a prosthetic ligament for use in a total knee replacementprosthesis.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Joint replacement prostheses commonly comprise two bone engagingcomponents that articulate via a bearing component. In a total kneereplacement prosthesis, the bone engaging components are a femoralcomponent, comprising an anterior surface with patella track and twofemoral condyles, and a tibia component, comprising a substantiallyplanar surface or tray and a post, keel or other stabilizing feature.The femoral and tibia components articulate via a bearing componentmounted on the tray of the tibial component. The bearing component maybe fully or partially fixed with respect to the tibial component, andcommonly comprises a single piece of high density polyethylene.

In order to more closely replicate the natural kinematics of the knee,it is desirable for a total knee replacement prosthesis to facilitate acombination of rolling, rotational and translational movement betweenthe femoral and tibial components of the prosthesis. This can beachieved in part by employing a “mobile” bearing component, having somefreedom of movement relative to the tibial component on which it issupported.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

According to a first aspect, there is provided a bearing component for ajoint replacement prosthesis, the component comprising: a first bearingelement; a second bearing element, and a linking element, operativelyconnecting the first and second bearing elements and permitting relativemotion there between.

The linking element provides a physical connection between the twobearing elements while still allowing relative motion between the twobearing elements. The linking element thus prevents dislocation ofeither bearing element in the event of distraction in either compartmentof the prosthesis in which the bearing element is employed.

The linking element may be flexible and may be elastic or resilient.Such a linking element thus affords a greater range of relative motionbetween the two bearing elements

The first and second bearing elements and the linking element may beintegrally formed. The linking element may comprise a polyethylenemembrane.

Alternatively, the linking element may comprise a fabric or polyestercord.

Respective ends of the linking element may be moulded into the first andsecond bearing elements. The linking element may extend into and throughbores that may extend at least partially through the bearing elements.

According to another aspect, there is provided a joint replacementprosthesis comprising first and second bone engaging components thatarticulate via the bearing component.

The prosthesis may be a knee replacement prosthesis, the first boneengaging component comprising a femoral component and the second boneengaging component comprising a tibial component.

The bearing component may be supported on the tibial component such thatrelative motion between the tibial component and first and secondbearing elements is enabled.

The articulation between the femoral component and the first and secondbearing elements may be at least partially spherical.

The tibial component may comprise at least one tray, on which thebearing component is supported, and a retaining element that extendsacross the tray, between the first and second bearing elements, passingover the linking element. The retaining element may thus preventdislocation of the entire bearing component in the event of bilateraldistraction.

The retaining element may comprise a bridge, under which the linkingelement of the bearing component passes, such that the bridge limits theextent of relative motion possible between the linking element and thetibial tray.

The retaining element may be removably connected to the tibial tray,thus facilitating assembly of the prosthesis.

The joint replacement prosthesis may further comprise an artificialligament.

The artificial ligament may extend between and be connected to thetibial and femoral components. The artificial ligament may engage theretaining element during at least part of its range of movement. Theretaining element may be adapted to support and/or deflect theartificial ligament and/or to change a line of action of the ligament.For example, the retaining element may comprise a pulley or have aligament support surface which has a recess or is waisted to align theligament and to prevent dislocation. The ligament support surface may becurved and/or chamfered and/or polished to reduce wear of the artificialligament.

The tibial component of the joint replacement prosthesis may comprisefirst and second bearing surfaces, operable to articulate with the firstand second bearing elements of the bearing component, the first bearingsurface being convex and the second bearing surface being noncon-convex.

The second bearing surface may be concave. The convex and concavebearing surfaces may be at least partially spherical.

According to another aspect, there is provided a method of making abearing component of the present invention comprising direct compressionmoulding ends of the linking element into the first and second bearingelements.

According to another aspect, there is provided a method of making abearing component comprising moulding the first and second bearingelements as a single piece and removing material from the area betweenthe first and second bearing components to define the linking element.

According to another aspect, there is provided a flexible linkingelement for use in preventing dislocation of mobile bearings in a totalknee replacement prosthesis.

The flexible linking element may connect first and second mobile bearingelements. Movement of the flexible linking element may be at leastpartially constrained by a retaining element.

According to another aspect, there is disclosed use of a flexiblelinking element to prevent dislocation of mobile bearings in a totalknee replacement prosthesis.

The flexible linking element may connect first and second mobile bearingelements. Movement of the flexible linking element may be at leastpartially constrained by a retaining element.

According to another aspect, there is provided a tibial component for aknee replacement prosthesis, the component comprising a lateralcompartment having a convex bearing surface and a medial compartmenthaving a non-convex bearing surface. The convex lateral bearing surfaceprovides increased stability to the lateral compartment of the joint,when the tibial component is assembled in a knee replacement prosthesis.

The bearing surface of the medial compartment may be concave. Such aconcave medial compartment provides greater stability and facilitates inreplicating the natural motion of the knee when the tibial component isassembled in a knee replacement prosthesis, primarily by reducing motionin the medial compartment of the knee. Combining a convex lateralcompartment with a concave medial compartment facilitates restoration ofthe natural motion of the knee.

Alternatively, the bearing surface of the medial compartment may beplanar.

The lateral and medial bearing surfaces may be part spherical and theradii of curvature of the lateral and medial bearing surfaces may besubstantially the same. The centre of curvature of each bearing surfacemay be anterior of the anterior/posterior centre line of the bearingsurface and on the medial/lateral centre line of the bearing surface.

The lateral and/or medial bearing surfaces may comprise modular surfacecomponents, operable to be connected to a tray component to form thetibial component. The modular surface components and tray component maycomprise cooperating fittings to facilitate connection and removal ofthe modular surface components.

According to another aspect, there is provided a kit of parts for atibial component of a knee prosthesis, the kit comprising a tibial trayand a plurality of surface components, operable to be removablyconnected to the tray, the surface components each comprising a bearingsurface, at least one surface component comprising a convex bearingsurface and at least one surface component comprising a concave bearingsurface.

According to another aspect, there is provided a knee replacementprosthesis comprising the bearing component and the tibial component.

It is known to implant an artificial ligament to replace a naturalligament which has become damaged. Conventional artificial ligaments areformed from strands or bundles of artificial fibres which may be wovenand/or aligned to form a flexible member which is substantially uniformin size and is resilient along its length.

A natural ligament exhibits high strength, toughness and resilience andretains these properties for many years. To date, it has been impossibleto match these properties using artificial fibres.

When implanted, artificial ligaments may be attached to existing bonetissue, provided the tissue at the attachment site is relatively intact.However, if surrounding bone tissue is diseased or damaged, it may benecessary to remove both the natural ligament and the adjacent bonetissue and replace them with prosthetic components.

Joint replacement operations commonly result in removal of at least oneligament. The functionality of the ligament is replicated as closely aspossible by one or more features of the replacement prosthesis (as forexample in the case of a cooperating cam and post in a total kneereplacement). However, it has proved extremely difficult to replicatethe natural kinematics of a joint without the presence of naturallyfunctioning ligaments. This is particularly evident in the case of theknee joint, which exhibits a complex movement that is highly dependentupon the interaction of ligaments with the articulating areas of bone.

According to another aspect, there is provided a joint replacementprosthesis comprising an artificial ligament, which is adapted toreplace a human or animal ligament, and a biasing element operativelycoupled to the artificial ligament to control the effective stiffness ofthe artificial ligament.

The biasing element may have a stiffness approximating that of a naturalligament that is to be replaced. In this manner, the biasing element mayassist in replicating the natural characteristics of the joint. Thebiasing element may have linear or non-linear stiffness characteristicswhich may be achieved by methods known in the art. The stiffness of thebiasing element may be in a range of 3 N/mm to 40 N/mm.

The biasing element may comprise one or more springs and/or one or moreelastic or elastomeric members and/or one or more Belleville washers.The biasing element may comprise a cylinder, tube, toroid, cone or loopof elastic or elastomeric material.

The biasing element may be a coil spring and may be a tension spring ora compression spring. Alternatively the biasing element may be a leafspring. The leaf spring may engage an abutment at a predeterminedposition in its range of movement to vary the effective stiffness of thespring.

The spring may be conical, so that it provides variable stiffness overits range of movement. It may, for example, be a conical coil spring.

The biasing element may be operatively coupled to the ligament at ornear one end only of the ligament. The biasing element may engage theligament via a bearing component.

According to another aspect, there is provided a joint replacementprosthesis comprising an artificial ligament, which is adapted toreplace a human or animal ligament, and a tensioning element operativelycoupled to the ligament for applying tension to the ligament.

The tensioning element may be operatively coupled to the ligament at ornear one end only of the ligament.

The prosthesis may further comprise a biasing element, which maycomprise an elastic element. The biasing element may act between thetensioning element and the ligament. The tensioning element may becoupled to the ligament via the biasing element. The biasing element mayengage the ligament via a bearing element.

The biasing element may be formed as in the previous aspect. Forexample, it may comprise a spring, which may be a compression spring.The spring may be of any form. For example it may comprise a coilspring, a leaf spring, a Belleville washer or an elastic or elastomericmember.

The biasing element may have a stiffness approximating that of a naturalligament that is to be replaced. The biasing element may have a linearstiffness characteristic. Alternatively, the biasing element may have anon-linear stiffness characteristic.

The ligament may be coupled to the tensioning element via an attachmentmeans. The attachment means may comprise an enlarged portion that isformed on the ligament and engages the tensioning element. For example,the enlarged portion may comprise a knot tied in the artificialligament.

The prosthesis may further comprise a bone engaging element forattachment to a bone. The tensioning element may act between the boneengaging element and the artificial ligament.

The tensioning element may be at least partially housed within the boneengaging element. The bone engaging element may comprise a stem, and thetensioning element may be at least partially housed within the stem.

The tensioning element may be adjustable and may be operable to adjustthe tension within the ligament. The adjustable tensioning elementtherefore enables the tension in the ligament to be adjusted in acontrolled manner, independently and controllably altering thecharacteristics of the ligament.

The prosthesis may further comprise adjustment means operable to adjustthe tension in the ligament. The tension may be adjusted by adjustingthe position of the tensioning element relative to the bone engagingelement. Thus tension in the ligament may be altered even after bothligament and bone engaging element have been implanted into a patient.

The adjustment means may comprise a threaded connection between thetensioning element and the bone engaging element. The tensioning elementmay comprise an external thread and the stem may comprise acorresponding internally threaded bore within which the tensioningelement is received.

The tensioning element may be configured to be screwed into or out ofthe bore to adjust the tension in the ligament.

The tensioning element may be accessed through an opening formed in thebone engaging element.

The prosthesis may further comprise a retaining element for limiting themotion of one or more bearing elements of the prosthesis, the retainingelement being adapted to engage an artificial ligament to thereby changea line of action of the ligament.

The prosthesis may comprise only part of a joint replacement prosthesis,which may be a knee replacement prosthesis.

The prosthesis may comprise at least part of a knee replacementprosthesis in which the bone engaging element comprises a tibialcomponent and the artificial ligament comprises a replacement anteriorcruciate ligament (ACL).

Artificial ligaments without suitable stiffness characteristics do notbalance with the other soft tissue, resulting in abnormal kinematics. Byusing a ligament with physiological stiffness, there will be mutualrespect with the retained soft tissue, allowing the joint to functionnormally.

The biasing element and/or tensioning element can protect the ligamentfrom excess load. It has been shown that the loads induced in anartificial ligament which is substantially inextensible are far inexcess of the ultimate tensile stress of a natural ligament. By allowingjust a small amount of extension in the artificial ligament, these loadsare reduced and the ligament and its attachment are protected.

The biasing element and/or tensioning element can enable the tension ofthe ligament to be balanced with other soft tissues. This eases theimplantation of an artificial ligament because fixation can be optimisedfirst before applying tension to a ligament. In embodiments with anadjustable tensioning element and/or with adjustable fixation of theligament to the femur a single size or limited range of sizes ofartificial ligament can be used on any patient. This reduces theinventory requirements for artificial ligaments.

According to another aspect, there is provided a joint replacementprosthesis comprising a bone engaging element having a stem which isadapted to project into a bone, and an artificial ligament, an end ofthe ligament extending into and being secured within the stem.

The prosthesis may comprise at least part of a joint replacementprosthesis, which may be a knee replacement prosthesis.

The prosthesis may comprise at least part of a knee replacementprosthesis in which the bone engaging element comprises a tibialcomponent and the artificial ligament comprises a replacement anteriorcruciate ligament (ACL).

The ligament may be secured within the stem of the bone engaging elementvia a tensioning element, the tensioning element being at leastpartially housed within the bone engaging element.

The ligament may be secured within the stem of the bone engaging elementvia a biasing element, the biasing element being at least partiallyhoused within the bone engaging element.

The ligament may be secured within the stem of the bone engaging elementvia both a tensioning element and a biasing element, the tensioningelement and the biasing element being at least partially housed withinthe bone engaging element.

According to another aspect, there is provided a prosthesis comprising abone engaging element and an artificial ligament, an end of theartificial ligament being secured to the body of the bone engagingelement, the prosthesis further comprising a ligament support, theligament support at least partially determining the line of action ofthe ligament.

The artificial ligament may be secured within a recess formed in thebody of the bone engaging element, and the ligament support may comprisea mouth of the recess. The mouth of the recess may be radiused orchamfered.

The ligament support may be shaped like a pulley. For example it may besubstantially cotton reel shaped or otherwise formed with a recess orwaisted portion. This recess or waisted portion is helpful in centringthe artificial ligament and reduces the possibility of dislocation.

The ligament support may project from a surface of the bone engagingelement, or may be formed on the retaining element described in previousaspects. Alternatively, the artificial ligament may be secured to thebone engaging element or to the retaining element.

The prosthesis may comprise at least part of a knee replacementprosthesis.

This aspect allows the direction of action of an artificial ligament tobe changed, in particular where a biasing element is used to control thestiffness of the assembly. For example, if the biasing element is housedwithin the tibial stem of a knee replacement prosthesis its line ofaction must be parallel to that stem. However, the line of action of theligament must be towards a point of attachment to the femoral componentor femur, the position of which changes through the range of motion. Theuse of a ligament support allows for the necessary change in thedirection of the line of action of the ligament.

According to another aspect, there is provided a method of implanting aprosthesis comprising first and second bone engaging elements, anartificial ligament and a biasing element, comprising: (a) operativelycoupling the biasing element to a first end of the artificial ligament;(b) operatively coupling the biasing element to the first bone engagingelement; (c) implanting the first and second bone engaging elements intobone tissue; (d) connecting a second end of the artificial ligament tothe second bone engaging element, (e) balancing tension within theartificial ligament.

According to another aspect, there is provided a method of implanting aprosthesis comprising first and second bone engaging elements, anartificial ligament and an adjustable tensioning element operativelycoupled to a first end of the artificial ligament and to the first boneengaging element, the method comprising: (a) assembling the first boneengaging element, adjustable tensioning element and artificial ligament;(b) implanting the first and second bone engaging elements into bonetissue; (c) connecting a second end of the artificial ligament to thesecond bone engaging element; (d) adjusting the position of theadjustable tensioning element until a predetermined tension within theartificial ligament is achieved.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a plan view of a bearing component

FIG. 2 is a plan view of an alternative bearing component

FIG. 3 is a plan view of a tibial tray component

FIG. 4 is a perspective view of the tibial tray component of FIG. 3

FIG. 5 is a perspective view of the bearing component and the tibialtray component

FIG. 6 is a perspective view of a partially assembled knee replacementprosthesis

FIG. 7 is a plan view of an embodiment of a tibial component

FIG. 8 is a sectional view along the anterior/posterior centre line XXof the component of FIG. 7.

FIG. 9 is a sectional view along the medial/lateral centre line YY ofthe lateral compartment of the component of FIG. 7.

FIG. 10 is a sectional view along the medial/lateral centre line ZZ ofthe medial compartment of the component of FIG. 7.

FIG. 11 is a perspective view of an embodiment of a tibial component

FIG. 12 is a sectional view of another embodiment of a tibial component

FIG. 13 is a sectional view of the component of FIG. 12 in analternative arrangement

FIG. 14 is a sectional view of an embodiment of a bearing component

FIG. 15 is a perspective view of a knee prosthesis having an artificialligament.

FIG. 16 is a partially sectioned side view of a knee prosthesis havingan artificial ligament secured via a biasing element.

FIG. 17 is a partial sectional view of the embodiment of FIG. 16.

FIG. 18 is a partially sectioned side view of a knee prosthesis havingan artificial ligament secured via a tensioning device.

FIG. 19 is a partial sectional view of the embodiment of FIG. 18.

FIG. 20 is a partial sectional view of a knee prosthesis having anartificial ligament secured via a tensioning device and a biasingdevice.

FIG. 21 is a partial sectional view of a knee prosthesis having anartificial ligament and a ligament support.

FIG. 22 is a perspective view of a partially assembled knee replacementprosthesis with a modified ligament support. Corresponding referencenumerals indicate corresponding parts throughout the several views ofthe drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

With reference to FIGS. 1 and 2, a bearing component 2 comprises firstand second bearing elements 4, 6 and a flexible linking element 8.Bearing component 2 is suitable for use as a meniscal bearing componentof a total knee prosthesis, the prosthesis comprising a tibialcomponent, a femoral component and the bearing component 2. Bearingelements 4, 6 of the bearing component are formed of moulded highdensity polyethylene and each comprise a distal bearing surface (notshown), that is shaped to articulate with a tibial tray, and a proximalbearing surface 10, 12, that is shaped to articulate with an associatedcondyle of a femoral prosthesis. The proximal bearing surfaces 10, 12may have any suitable shape appropriate for the chosen femoral componentwith which the bearing component is to articulate.

The linking element 8 may be a separate cord 14, connected to thebearing elements 4, 6, as illustrated in FIG. 1. The cord 14 may be ofany appropriate shape or material. For example, it may comprise a wovenflexible fabric or polyester cord. The cord 14 passes throughappropriately dimensioned bores 18, 20 that extend across the width ofthe bearing elements 4, 6 such that ends of the cord 14 extend out ofopposite sides of each bearing element 4, 6. A knot 22 is tied in eachend of the cord 14 to prevent the cord 14 passing back through the bores18, 20. The two bearing elements 4, 6 are thus connected together.Relative movement between the bearing elements 4, 6 is possible, as thecord 14 is flexible. In alternative embodiments, the knot 22 may bereplaced by a ball or other protruding feature that prevents passage ofthe ends of the cord back through the bores 18, 20. In anotherembodiment, as illustrated for example in FIGS. 5 and 6, the cord 14 maybe moulded into the bearing elements at the time of manufacture.

In an alternative embodiment, the linking element 8 may be an integralcomponent 16 of the bearing elements 4, 6, as illustrated in FIG. 2. Thelinking element may for example comprise a thin polyethylene membrane 16that connects the bearing elements 4, 6 while permitting relative motionthere between. The membrane 16 may be moulded into the bearing elements4, 6 or may be formed during manufacture of the bearing elements byforming the bearing component 2 as a single component and then removingmaterial from the component so as to define the two bearing elements 4,6, leaving only the thin membrane of material 16 connecting the twoelements together.

With reference to FIGS. 3 to 6, a tibial component suitable for use withthe bearing component 2 comprises a tray 30 and a bridge element 50. Thebridge element 50 is omitted in FIGS. 3 to 5 for clarity. The tray 30 isformed of a suitable biocompatible metal, such as stainless steel orcobalt chromium molybdenum. The tray comprises a distal surface 32,which engages a resected tibial bone surface, and may comprise a keel orother stabilising feature (not shown). The tray 30 further comprises aproximal surface 34 that articulates with the distal surfaces of thebearing elements 4, 6 of the bearing component 2, when the bearingcomponent 2 and tibial component are assembled. The tibial componentcomprises a lateral compartment 36 and a medial compartment 38. Thelateral and medial compartments may each comprise bearing surfaces 40,42 with which the bearing elements 4, 6 of the bearing component 2articulate. The bearing surfaces 40, 42 may be planar, and thearticulation may comprise sliding articulation.

With reference particularly to FIG. 6, the bridge element 50 comprises abeam 52 and two supporting legs 54, 56. The beam 52 extendssubstantially parallel to the proximal surface 34 of the tibial tray 30in an anterior/posterior direction and spans substantially the entirewidth of the tray 30. The supporting legs 54, 56 are positionedproximate the anterior and posterior edges of the tray, substantiallyequidistant of the medial and lateral edges of the tray 30. The bridgeelement 50 thus divides the tray 30 into its lateral and medialcompartments 36, 38 and defines a passage 60 there between. A proximalsurface of the beam 52 may comprise a threaded blind bore andcooperating screw (not shown), suitable for attaching one end of anartificial ligament (not shown) to the bridge element 50. Alternativemeans of connecting an artificial ligament may also be used.

The bridge element 50 is formed of any suitable biocompatible metal andmay be formed integrally with the tibial tray 30. Alternatively, thebridge element may be connected to the tray 30 in any appropriatemanner. Preferably, the bridge element 50 is removably connected to thetray 30, facilitating assembly of the final prosthesis. Alternatively,the bridge element 50 may be fixedly connected to the tray 30 at thetime of manufacture.

The bearing component 2 and tibial tray 30 may be assembled at the timeof manufacture or immediately prior to implantation. When the bearingcomponent 2 and tibial tray 30 are assembled, as illustrated in FIG. 5,the bearing elements 4, 6 of the bearing component 2 each rest on arespective bearing surface 40, 42 of the tray 30. The linking element 8of the bearing component 2 connects the bearing elements 4, 6 together.When the bridge element 50 is assembled with the tray 30 to form thetibial component, as illustrated in FIG. 6, the linking element 8 of thebearing component extends under the beam 52 of the bridge element,through the passage 60 to connect the bearing element 4, 6. If thebridge element 50 is integrally formed with the tibial tray 30, the tray30, bridge element 50 and bearing component 2 may be assembled at thetime of manufacture. Alternatively, if the bridge element 50 is notintegrally formed with the tray 30 but may be connected to the trayafter manufacture, then the tray 30, bridge element 50 and bearingcomponent 2 may be assembled at any time prior to implantation.

In use, the tibial tray 30, bridge element 50 and bearing component 2are assembled and implanted by mounting the tibial component onto aresected proximal surface of a tibia. A femoral component is attached toa distal surface of a femur and the resurfaced joint is assembled. Thebearing elements 4, 6 cushion the articulation between the tibial trayand the condyles of the femoral component. The bearing elements 4, 6 aremobile but are retained safely within the joint by the combined actionof the linking element 8 and the bridge element 52. In the event ofdistraction of the joint in either the lateral or medial compartments,the bearing element in the distracted compartment is held within thejoint by its connection to the other bearing element via the linkingelement 8. In the event of bilateral distraction, the bearing component2 is held within the joint space by the bridge 52. The passage 60defined by the bridge element 50 is not sufficiently large to allowpassage of either of the bearing elements 4, 6, so dislocation of thebearing component 2 is prevented.

With reference to FIGS. 7 to 11, a tibial component 100 of a kneeprosthesis comprises a distal surface 102, that is operable to engage aresected surface of a tibia, and a proximal surface 104, that isoperable to engage one or more bearing components (not shown). Thecomponent may further comprise a post, keel or other stabilising feature(not shown) that extends from the distal surface and provides stabilityto the component 100 when implanted. The component comprises a lateralcompartment 106, including a lateral portion of the proximal and distalsurfaces 102, 104, and a medial compartment 108, including a medialportion of the proximal and distal surfaces 102, 104. The lateral andmedial compartments are separated by a central region 114.

Each of the lateral and medial compartments comprises a proximal bearingsurface 110, 112. The lateral proximal bearing surface 110 is convex ordomed, having a part spherical surface with a radius of curvature R_(l).The centre of curvature of the lateral bearing surface 110 is justanterior of the anterior/posterior centre line)(X of the tibialcomponent 100, and is approximately on the medial/lateral centreline YYof the lateral compartment 106 of the tibial component 100. The medialproximal bearing surface 112 is concave or dished, having a partspherical surface with a radius of curvature R_(m), which is preferablyof a similar length to the lateral bearing surface radius of curvatureR_(l). The centre of curvature of the medial bearing surface 112 is alsojust anterior of the anterior/posterior centre line XX of the tibialcomponent 100, and is approximately on the medial/lateral centreline ZZof the medial compartment 108 of the tibial component 100.

Kinematic analysis of a patient may be employed to determine the heighth_(m), h_(l) of each compartment of the tibial component 100.Alternatively, the heights h_(l), h_(m) of the lateral and medialcompartments may be selected according to the natural positions of thelateral and medial bearing surfaces of the healthy tibia.

With reference to FIGS. 12 and 13, another embodiment of tibialcomponent 200 comprises a tibial tray 270 and at least three modularsurface components 280, 282, 284. The tibial tray 270 comprises a distalsurface 202, that is operable to engage a resected surface of a tibia,and a proximal surface 204, that is operable to engage one or moremodular surface components. The proximal surface 204 comprises lateraland medial support surfaces 208, 210, each of which may comprise aconnection feature, for example a recess 212, operable to cooperate witha corresponding connection feature on a corresponding modular surfacecomponent, as described below.

The at least three modular surface components comprise a domed lateralsurface component 280, having a convex part spherical proximal bearingsurface 210 as described above with respect to tibial component 100, adished medial surface component 282, having a concave part sphericalproximal bearing surface 212 as described above with respect to tibialcomponent 100, and a planar medial surface component 284, having aplanar proximal bearing surface 213. The tibial component 200 may alsocomprise a lateral planar surface component having a planar proximalbearing surface (not shown). Each modular surface component 280, 282,284 comprises a distal bearing surface that articulates with acorresponding support surface 208, 210 of the tibial tray 270. Thedistal bearing surfaces of the modular surface components 280, 282, 284each comprise a connection feature, for example a lug 290, operable tocooperate with the corresponding connection feature on the correspondingtibial support surface 208, 210.

The tibial component 200 may be provided as a kit of parts comprising atibial tray 270 and a selection of modular surface components 280, 282,284, enabling a surgeon to select an appropriate combination of domed,dished and planar bearing surfaces to suit a particular patient.

Both embodiments of tibial component 100, 200 are operable to be used incombination with appropriately shaped bearings (not shown). The bearingscomprise individual medial and lateral bearings, each having a proximalfemoral bearing surface that is shaped to articulate with a femoralcomponent of a knee prosthesis, and a distal tibial bearing surface thatis shaped to articulate with the appropriate one of the lateral ormedial bearing surfaces of the tibial component 100, 200. A combinationof domed lateral and dished medial bearing surfaces on the tibialcomponent 100, 200 provides increased stability to individual meniscalbearings, and facilitates in recreating the natural motion of the knee.

The tibial components 100, 200 described with reference to FIGS. 7 to13, may used in combination with the bearing component 2 described withreference to FIGS. 1 to 6. A bridge element 50 as described above withrespect to FIG. 6 may be mounted in the central region 114 of the tibialcomponent 100, 200 of FIGS. 7 to 13. If the bearing component 2 is to beused in connection with a tibial component 100, 200 as described withreference to FIGS. 7 to 13, the distal bearing surfaces of the bearingelements 4, 6 of the bearing component 2 are shaped to articulate with,for example, the convex and concave bearing surfaces 210, 212 of thetibial component 200. An example of a bearing component 102 for use witheither of the tibial components 100, 200 of FIGS. 7 to 13 is illustratedin sectional view in FIG. 14. The bearing component 102 compriseslateral and medial bearing elements 106, 104, each of which comprises aproximal bearing surface 110, 112, shaped to articulate with anassociated femoral condyle. The bearing elements 104, 106 furthercomprise distal bearing surfaces 113, 111, each of which is shaped toarticulate with a corresponding proximal tibial bearing surface 210,212. Thus, the distal bearing surface 113 of the lateral bearing element106 is part spherical concave, having a radius of curvaturesubstantially equal to the radius of curvature R_(l) of the lateralbearing surface 210 of the tibial component. Similarly, the distalbearing surface 111 of the medial bearing element 104 is part sphericalconvex, having a radius of curvature substantially equal to the radiusof curvature R_(m) of the medial bearing surface 212 of the tibialcomponent.

It will be understood by one skilled in the art that any aspect of anyof the embodiments described herein may be used in combination with anyother aspect of any of the embodiments described herein.

Referring to FIG. 15, a knee prosthesis 1002 comprises a tibialcomponent 1004 having a tibial tray 1006 integrally formed with a stem1008, a femoral component 1010 and a pair of bearing components 1012,1013. The bearing components 1012, 1013 separate the tibial component1004 and femoral component 1010, and are formed with proximal and distalbearing surfaces which engage corresponding bearing surfaces 1014, 1015,1016 on the tibial tray 1006 and on the femoral component 1010. Thesevarious bearing surfaces enable the tibial component 1004 to rotate andtranslate relative to the femoral component 1010. The bearing components1012, 1013 may be meniscal bearing components, rotational platformbearing components, or fixed bearing components and may be joinedbearing components which may be shaped and may articulate in accordancewith the embodiments of FIGS. 1 to 14.

FIGS. 16 and 17 illustrate an embodiment of prosthesis 102, in which anartificial ligament 1118 is connected at one end 1121 to the femoralcomponent 1110, and at the other end 1123 to a biasing element 1140mounted in the stem 1108 of the tibial component 1104. The biasingelement 1140 engages the ligament 1118 via a bearing element 1144. Thebiasing element 1140 and bearing element 1144 are both received within abore 1126 formed in the stem 1108. The bore 1126 opens onto the bearingsurface 1114 of the tibial tray 1106 at a mouth 1130. The mouth 1130extends partially into the bore 1126 to define an internal annularshoulder 1154 having an annular bearing surface 1156. The mouth 1130 issmooth, widening to accommodate the artificial ligament 1118 with someplay. The mouth may be radiused or chamfered. The artificial ligament1118 extends into the bore 1126 through a space 1127, defined betweenthe bearing components 1112, 1113, so that the artificial ligament 1118substantially does not interfere with the bearing components 1112 duringnormal articulation of the prosthesis.

Any convenient means of connection of the end 1121 of the ligament 1118to the femoral component 1110 is contemplated. For example, a boss orpeg 1119 may be formed on the femoral component for attachment of theligament 1118. The end 1121 of the ligament 1118 may be folded over andglued, sewn or otherwise fixed to form a loop (not shown).Alternatively, a hole or eye may be formed in the end 1121 of theligament 1118. The artificial ligament may then be secured to the boss1119 by passing the loop or eye over the boss 1119. The boss 1119 mayhave an enlarged head and narrower stem to encourage stable fixation ofthe ligament once attached to the boss 1119.

With reference also to FIG. 18, the other end 1123 of the artificialligament 1118 is attached to the biasing element 1140 via the bearingelement 1144. Any convenient means of connection between the end 1123 ofthe ligament 1118 and the bearing element 1144 is contemplated. Forexample, the end 1123 of the ligament 1118 may pass wholly orsubstantially through the bearing element 1144 and be prevented frompassing back through the bearing element 1144 by a stop 1134. The stop1134 may take the form of an enlarged body, for example a spherical body(as illustrated in FIG. 16), a cylinder, or any other appropriate form.Alternatively, the stop may comprise a knot 1135 formed in the end 1123of the ligament 1118 (as illustrated in FIG. 17). In an alternativeembodiment (not shown) the end 1123 of the ligament 1118 may be attacheddirectly to the bearing element 1144 without passing through the body ofthe bearing element 1144.

The biasing element 1140 comprises a resilient element 1142. In theillustrated embodiment, the resilient element 1142 is a coiledcompression spring 1146 and the bearing element 1144 is a plate 1148.However, the resilient element may consist of or comprise anyappropriate spring or springs, for example a Belleville washer or anelastic or elastomeric member. An appropriate bearing element may beselected according to the choice of resilient element.

As illustrated particularly in FIG. 18, the spring 1146 and bearingplate 1148 are received within the bore 1126 of the stem 1108. Theartificial ligament 1118 extends into the mouth 1130 of the bore 1126,through the coil spring 1146 and through a passage 1150 formed in thebearing plate 1148. A stop 1134 or knot 1136 prevents the ligament 1118passing back through the passage 1150 as described above. When tensileforces are applied to the ligament 1118, the knot 1135 or stop 1134bears against an adjacent surface of the plate 1148, forcing an oppositesurface of the plate 1148 to engage and compress the spring 1146 againstthe annular bearing surface 1152 of the shoulder 1150.

The spring 1146 assists in replicating the natural stiffness of theligament that is to be replaced. The characteristics of the spring aretherefore selected to be similar to those of the natural ACL.

Referring to FIGS. 18 and 19, in a further embodiment of prosthesis1202, an artificial ligament 1218 is connected at one end 1221 to thefemoral component 1210, and at the other end 1223 to a tensioningelement 1220 mounted in the stem 1208 of the tibial component 1204. Thetensioning element 1220 is cylindrical and formed with an externalthread 1222 which engages an internal thread 1224 formed in a bore 1226in the stem 1208.

As in the embodiment of FIGS. 16 and 17, any convenient means ofconnection of the end 1221 of the ligament 1218 to the femoral component1210 is contemplated. For example, a boss or peg 1219 may be formed onthe femoral component for attachment of the ligament 1218. The end 1221of the ligament 1218 may be folded over and glued, sewn or otherwisefixed to form a loop (not shown). Alternatively, a hole or eye may beformed in the end 1221 of the ligament 1218. The artificial ligament maythen be secured to the boss 1219 by passing the loop or eye over theboss 1219. The boss 1219 may have an enlarged head and narrower stem toencourage stable fixation of the ligament once attached to the boss1219.

The other end 1223 of the artificial ligament 1218 is attached to thetensioning element 1220. Again, as in the embodiment of FIGS. 16 and 17,any convenient means of connection between the end 1223 of the ligament1218 and the tensioning element 1220 is contemplated. For example, theend 1223 of the ligament 1218 may pass wholly or substantially throughthe tensioning element 1220 and be prevented from passing back throughthe tensioning element 1220 by a stop 1234. The stop 1234 may take theform of an enlarged body, for example a spherical body (as illustratedin FIG. 18), a cylinder, or any other appropriate form. Alternatively,the stop may comprise a knot 1235 formed in the end 1223 of the ligament1218 (as illustrated in FIG. 19). In an alternative embodiment (notshown) the end 1223 of the ligament 1218 may be attached directly to thetensioning element 1220 without passing through the body of thetensioning element 1220.

The bore 1226 in which the tensioning element 1220 is received opensonto the bearing surface 1214 of the tibial tray 1206 at a mouth 1230.The mouth 1230 is smooth, widening to accommodate the artificialligament 1218 with some play. The mouth may be radiused or chamfered.The artificial ligament 1218 extends into the bore 1226 through a space1227, defined between the bearing components 1212, 1213, so that theartificial ligament 1218 substantially does not interfere with thebearing components 1212 during normal articulation of the prosthesis. Inthe case of a monoblock bearing component (not shown) a suitable openingis formed to allow passage of the artificial ligament and to minimisewear or abrasion of the ligament 1218 during movement.

As illustrated in FIG. 19, the tensioning element 1220 may have asubstantially spherical recess 1229 in its end 1231 closest to the freeend 1228 of the stem 128. A passage 1232 extends from a base of therecess through the tensioning element towards the mouth 1230 in thetibial tray. The passage 1232 is large enough to receive the loop, eyeor other fixation feature at the end 1221 of the ligament 1218, but istoo small to allow the stop 1234 to pass through. Alternatively, asillustrated in FIG. 20, the tensioning element may simply comprise apassage 1232, the stop or knot 1234, 1235 engaging against a surface1237 of the tensioning element 1220.

With reference to FIG. 21, a further embodiment of knee prosthesiscombines features of the last two embodiments. The prosthesis 1302comprises a tensioning element 1320, substantially as described withreference to the embodiment of FIGS. 18 and 19, and a biasing element1340, substantially as described with reference to the embodiment ofFIGS. 16 and 17. The biasing element 1340 acts between the ligament 1318and the tensioning element 1320, as opposed to the shoulder 1350 of theembodiment of FIGS. 16 and 17. The biasing element 1340 comprises aresilient element 1342, which engages the ligament 1318 via a bearingelement 1344. In the illustrated embodiment, the resilient element 1342is a coiled compression spring 1346 and the bearing element 1344 is aplate 1348. However, the resilient element may be any appropriate springor springs, for example a Belleville washer or an elastic or elastomericmember. An appropriate bearing element may be selected according to thechoice of resilient element.

As illustrated in FIG. 21, the spring 1346 and bearing plate 1348 arereceived within the bore 1326 of the stem 1308 beneath the tensioningelement 1320. The artificial ligament 1318 extends through the passage1332 in the tensioning element, through the coil spring 1346 and througha passage 1350 formed in the bearing plate 1348. A stop 1334 or knot1336 is formed on the end 1323 of the ligament 1318 as described above.The knot 1335 or stop 1334 prevents the ligament 1318 passing backthrough the passage 1350. When tensile forces are applied to theligament 1318, the knot 1335 or stop 1334 bears against an adjacentsurface of the plate 1348, forcing an opposite surface of the plate 1348to engage and compress the spring 1346 against the adjacent surface ofthe tensioning element 1320.

The spring assists in replicating the natural stiffness of a ligament.The characteristics of the spring are selected accordingly to be similarto those of the natural ACL.

Implantation of the prosthesis of the present invention will bedescribed with reference to the embodiment of FIG. 20. However, it willbe understood that corresponding techniques may be employed for allembodiments disclosed herein.

In use of the prosthesis 1302, the femoral component 1310 is implantedinto a distal end of a femur (not shown) and the tibial component 1304is implanted into a proximal end of a tibia (not shown), such that thestem 1308 is located in the intramedulary canal of the tibia, and thetibial tray 1306 rests on the resected proximal end of the tibia. Theappropriate bearing component(s) are placed between the femoralcomponent 1310 and the tibial component 1304.

The artificial ligament 1318, compression spring 1346 and bearing plate1348, tensioning element 1320 and tibial component 1304 are preassembledprior to implantation. The ligament 1318 is connected to the tensioningelement 1320 by passing the end 1321 of the ligament 1318 through thepassage 1332 via the passage 1350 in the bearing plate 1348 and thespring 1346 and feeding the ligament 1318 through the passage 1332 untilthe stop 1334 or knot 1335 engages a surface of the bearing plate 1348.The tensioning element 1320 is then screwed to an appropriate depth intothe bore 1326 in the tibial component 1304 to achieve initial tensioningof the ligament 1318 when fully connected.

The femoral and tibial components 1310, 1304 are then implanted usingstandard techniques. Once the tibial component 1304 is implanted, thefree end 1321 of the ligament 1318 projects through the mouth 1330 inthe tibial tray towards the femoral component 1310. The appropriatebearing components are then placed between the femoral component 1310and the tibial component 1304 in a known manner.

The end 1121 of the ligament 1118 is then attached to the femoralcomponent 1110 by passing the loop or eye over the boss 1119.

The joint is then examined to determine whether the tension in theartificial ligament 1318 is balanced with the tension in the retainedposterior cruciate ligament (PCL). If the tension in the artificialligament 1318 is balanced with that in the PCL, the implantationprocedure is complete. If the tension in the artificial ligament 1318 isnot balanced with that in the PCL, the position of the tensioningelement 1320 within the bore 1326 is adjusted, so as to increase orreduce the tension applied to the ligament 1318. A tool (not shown) maybe inserted through the mouth 1330 to engage a drive formation (notshown) formed on the tensioning element 1320. By rotating the tool, thetensioning element 1320 is rotated and moves axially along the internalthread in the bore 1326, thereby adjusting the tension in the artificialligament 1318.

With reference to FIGS. 17 to 20, the embodiments of the presentinvention may further comprise a ligament support 1160 that is operableto change the line of action of the artificial ligament 1118. Theligament support may comprise a section of or a projection from themouth 1130 of the bore 1126 in the tibial component 1114. Alternatively,as shown in FIG. 21, the ligament support may comprise a lug 1070 thatprojects from the surface 1014 of the tibial tray 1006. The lug may beintegrally formed with or connected to the tibial tray 1006.

FIG. 22 shows a knee prosthesis comprising a tibial component 1004having a tibial tray 1006 integrally formed with a stem 1008. A pair ofbearing components 2012, 2013 separate the tibial component 1004 from afemoral component (not shown) and are formed with bearing surfaces whichengage a corresponding bearing surface 1014 on the tibial tray 1006. Thebearing components 2012, 2013 are interconnected by a linking element2008, as described with reference to the embodiment of FIGS. 1 and 2. Aretaining element in the form of bridge element 2050 is fixed to thetibial tray 1006 to limit the motion of the linking element 2008. Thebridge element 2050 comprises a beam 2052 and two supporting legs 2054,2056. The beam 2052 extends substantially parallel to the proximalsurface of the tibial tray 1006 in an anterior-posterior direction andspans substantially the entire width of the tibial tray 1006. Thesupporting legs 2054, 2056 are positioned approximately at the anteriorand posterior edges of the tray 1006, substantially equidistantly at themedial and lateral edges of the tray 1006. The bridge element 2050 thusdivides the tibial tray 1006 into its lateral and medial compartmentsand defines a passage 2060 therebetween, which accommodates the linkingelement 2008.

An artificial ligament (not shown) is connected to a biasing element(not shown) housed in the stem 1008. As in previous embodimentsincorporating a ligament, the ligament passes out of an opening 1030 inthe tibial tray 1006 and abuts a side of the beam 2052 of the bridgeelement 2050. It will be appreciated that the engagement of the ligamentwith the side of the bridge element 2050 causes a deflection of theligament and a change in the line of action of the ligament. In order toavoid fretting or other wear related damage of the ligament in use, thebridge element 2050 is provided with a recess or chamfer 2051 whichhelps to locate the ligament, avoids dislocation and provides a smoothsurface of engagement between the bridge element 2050 and the ligament.Thus, the bridge element 2050 has the dual function of limiting themotion of the bearing components 2012, 2013 and acting as a ligamentsupport to change the line of action of the artificial ligament. Inalternative embodiments not illustrated, the bridge element 2050 maycomprise a pulley or may be provided with a projection or boss to assistin aligning the ligament and preventing dislocation. Furthermore theligament support surface formed on the bridge element 2050 may bepolished or otherwise surface finished to reduce wear of the artificialligament.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

The invention claimed is:
 1. A prosthesis comprising: a tibial trayincluding a body defining a bore having a mouth, the bore opening onto abearing surface extending perpendicular to the mouth; a femoralcomponent; an artificial ligament coupled to the tibial tray and thefemoral component; and a ligament support configured to at leastpartially determine a line of action of the artificial ligament, whereinthe ligament support comprises a lug attached to the bearing surface andprojecting to a position over the mouth to prevent the artificialligament from touching the mouth.
 2. The prosthesis of claim 1, whereinthe artificial ligament is secured within the bore.
 3. The prosthesis ofclaim 1, wherein the mouth of the bore is chamfered.
 4. The prosthesisof claim 1, wherein the artificial ligament is secured to a surface ofthe bore.
 5. The prosthesis of claim 1, further comprising an adjustabletensioning element adjustably positionable within the bore andconfigured to adjust a tension on the artificial ligament.
 6. Theprosthesis of claim 5, further comprising a biasing element positionedwithin the bore and configured to provide a biasing force on theartificial ligament.
 7. The prosthesis of claim 6, further comprising abearing element positioned within the bore and adjacent the biasingelement and configured to engage an end of the artificial ligament.
 8. Aprosthesis comprising: a tibal component having a body that defines abore formed into the body, the body further defining a bearing surfaceextending perpendicular to the recess; an artificial ligament configuredto be secured within the recess; and a lug extending from the bearingsurface and arranged to support the artificial ligament, the lugextending from the bearing surface to a position over the recess toprevent the artificial ligament from touching a mouth of the recess,wherein the lug at least partially determines the line of action of theligament.
 9. The prosthesis of claim 8, wherein the mouth of the recessis chamfered.
 10. The prosthesis of claim 8, further compromising afemoral component configured to be secured to the artificial ligament.11. The prosthesis of claim 8, further compromising an adjustabletensioning member adjustably positioned within the recess and configuredto adjust a tension on the artificial ligament.
 12. The prosthesis ofclaim 11, further compromising a biasing element positioned within therecess and adjacent the tensioning member.
 13. The prosthesis of claim12, further compromising a bearing element positioned in the bore andadjacent the biasing member.